The manufacture of shaped articles of this type is difficult and expensive. It is virtually impossible by prior art methods to densify pure boron nitride powder to a high degree without the concomitant use of sintering aids. The sintering aids make it impossible to provide shaped articles having a density approaching the density of pure boron nitride (2.27 g/cm.sup.3).
The best results known hitherto with respect to the highest possible densification are achieved by hot die pressing at temperatures of from 1700.degree. to 2200.degree. C. at a die pressure of from approximately 10 to 30 MPa, boron nitride powder containing boric oxide having proved especially useful. Owing to the boric oxide content, the shaped articles thus obtained have poor high-temperature properties.
To improve the high temperature properties, the oxygen content of the boron nitride shaped articles manufactured by hot-die-pressing is reduced by washing out the boric oxide after sintering at from 1600.degree. to 2100.degree. C. (see U.S. Pat. No. 3,660,027 and U.S. Pat. No. 3,734,997, corresponding to DE-AS No. 20 21 952). The process, is unusually expensive and also is not useful for the treatment of relatively large shaped articles. Washing out the boric oxide to reduce the boric oxide content of relatively small shaped articles requires up to 44 days.
Regardless of the type of sintering aids used, shaped articles manufactured from hexagonal boron nitride powder by hot-die-pressing exhibit a directional grain growth and an anisotropic microstructure due to the biaxial application of pressure at a high temperature so that the properties of the articles are direction-dependent. The degree of anisotropy increase as higher degrees of densification are achieved.
Furthermore, it is generally known that, in addition to the customary hot-die-pressing using biaxial application of pressure, isostatic hot-pressing, with multidirectional application of pressure using an inert gas pressure-transfer medium can also be used. In isostatic hot-pressing, it is necessary to provide the powder to be densified or the article preshaped therefrom having open pores that is, having pores open to the surface, with a gas-tight casing to prevent the gas used as a pressure-transfer medium from penetrating into the article and interfering with densification.
Suitable casings for isostatic hot pressing of boron nitride powder having an excess of boron or nitrogen, are made of titanium or zirconium. The iron, chromium or nickel casings customarily used for isostatic hot-pressing form, with the excess boron or nitrogen, compositions which have melting points that are considerably lower than the melting points of the metal casing material. The casings become molten and no longer gas-tight and are penetrated by the pressure-transfer medium. It should therefore be possible to subject boron nitride powders of the type mentioned to the isostatic hot-pressing process at temperatures of from 1200.degree. to 1600.degree. C. under a pressure of from 20 to 300 MPa, but it must be pointed out expressly that under those conditions, boron nitride articles having high density have not been obtained (see U.S. Pat. No. 4,007,251).
Instead of the metal casings, preshaped articles may also be placed in prefabricated glass casings and the space between casing and article can be filled additionally with a glass powder that has a higher softening temperature than that of the casing itself (see Great Britain Patent Specification No. 1,529,966 corresponding to DE-PS No. 26 01 294).
Furthermore, some experimental work is known relating to the preparation of binder-free polygranular BN shapes from the technical report "Development and Evaluation of Hot Isostatically compacted Boron Nitride" of M. C. Brockway et al, AD 709 620 of July 1970, Battelle Memorial Institute, Columbus Laboratory, wherein commercial BN powders, purified by means of different purification techniques, were hydrostatically cold-pressed to form green bodies, which were then inserted into tantalum containers, sealed vacuum-tight by electron beam welding, and hot isostatically compacted at temperatures of from 1650.degree. C. to 2480.degree. C. under pressures of 207 MPa (30,000 p.s.i.) resp. 103 MPa (15,000 p.s.i.) for 1 to 3 hours.
It can be seen from the discussion of the experimental results that the achieved densities were in the range of commercial hot-die-pressed BN bodies. The data did not show a general increase in density with temperature, presumably due to gross grain growth at high temperatures. It was also stated that improved purity of the starting powders did not improve the properties such as density and mechanical strength, although the oxygen contents were below the limit of the best commercial hot-die-pressed BN, which usually is about 1 percent. Furthermore, it was stated that the samples were not completely isotropic, i.e., that obviously a directional grain growth could not be suppressed despite the multidirectional application of pressure. X-ray diffraction data disclosed a substantial amount of anisotropy in the articles which appeared to decrease as the length to diameter ratio of the article increased.
The critical parameters, which are necessary for manufacturing shaped articles consisting of boron nitride by means of the isostatically hot pressing procedure to obtain improved properties namely, high density combined with a substantially isotropic microstructure, were not noted by the authors of this report.
The object of the invention is therefore to provide dense shaped articles consisting of boron nitride having improved properties, which are manufactured in a simple manner without expensive after-treatment, by densification of pure boron nitride powder without the concomitant use of sintering aids.